1. Field of the Invention
The present invention relates to a phosphor suitable for a display device such as a cathode ray tube efficiently light-emitting at excitation by electron beam, ultraviolet ray or the like. Particularly, the present invention relates to a phosphor capable of improving a packing density of a phosphor in a phosphor film coated by a slurry coating method, a paste coating method or the like, improving a dispersibility of a phosphor and forming a dense phosphor film, and also capable of enhancing a luminance of a phosphor film and forming an image excellent in color purity and contrast.
2. Prior Art
A phosphor screen of a display device such as a cathode ray tube, is provided with a black matrix (hereinafter referred to as "BM") comprising a black non-luminant material on the inside surface of a face plate (glass-made panel) to improve a contrast, and a precoat layer comprising polyvinyl alcohol (PVA) or the like is formed on the black matrix (BM) to get the face plate well matched with a phosphor slurry. Thereafter, each phosphor slurry of three colors of G (green), B (blue) and R (red) is coated thereon in order and a phosphor film is formed by photographic printing method (exposure and development).
A "fogging" phenomenon occurring during this phosphor film-forming step deteriorates a color purity and a contrast of a phosphor screen. The above fogging phenomenon is classified as illustrated below.
(1) "fogging on BM": A phenomenon of deteriorating a contrast by a phosphor remained and deposited on a black matrix (BM); PA0 (2) "fogging on glass": A phenomenon of deteriorating a color purity of a phosphor film formed on a surface having a previously coated phosphor remained and deposited; and PA0 (3) "cross contamination": A phenomenon of deteriorating color purity of a previously coated color phosphor film due to the presence of phosphor particles coated thereafter. PA0 (1) A phosphor characterized by having a surface coated with a metal alkoxide containing a quaternary ammonium salt. PA0 (2) The phosphor as defined in the above paragraph (1), wherein the metal alkoxide has the following general formula: ##STR1## R.sub.1 to R.sub.3 : a C.sub.1 -C.sub.24 alkyl or allyl group R.sub.4 : a C.sub.1 -C.sub.4 alkyl group PA0 (3) The phosphor as defined in the above paragraph (2), wherein M is a Si element. PA0 (4) The phosphor as defined in any of the above paragraphs (1) to (3), wherein the phosphor surface is coated with an anionic organic compound and/or inorganic compound and then with the metal alkoxide. PA0 (5) A phosphor characterized by having a surface coated with nonionic or anionic compound particles and a quaternary ammonium salt-containing compound. PA0 (6) The phosphor as defined in the above paragraph (5), wherein the anionic compound is a silicon compound and/or carboxy-modified acrylic resin. PA0 (7) The phosphor as defined in the above paragraph (5), wherein the nonionic compound is polystyrene resin. PA0 (8) The phosphor as defined in any of the above paragraphs (1) to (7), wherein the phosphor surface has an isoelectric point of electrokinetic potential (zeta (.zeta.) potential) of at most 7, and is charged with a blow-off electric charge of at least 5 .mu.C/g when contacted with beads of a particle size of from 200 to 800 .mu.m having a surface coated with polyvinyl alcohol.
For example, "B/G" cross contamination means B contamination on a G phosphor film, and "R/G" cross contamination means R contamination on a G phosphor film.
It is therefore demanded to improve phosphor particles so as to reduce "fogging", thereby improving a color purity and a contrast, and to improve a packing density of a phosphor, thereby improving a luminance of a phosphor film.
In order to solve the above-mentioned problems caused by a fogging phenomenon and a phosphor packing density of a phosphor film, the present inventors have studied to chemically treat a phosphor surface with various surface-treating materials, and have discovered that the phosphor packing density and the fogging phenomenon are related to an isoelectric point of zeta potential of the surface-treated phosphor surface and a blow-off charge generated when contacted with beads covered with PVA, and that the above problems can be solved by coating the phosphor with a specific surface treating material.
Thus, the present invention is to provide a phosphor wherein a packing density for forming a phosphor film is improved and a fogging phenomenon can be prevented, by treating a phosphor surface with a specific material.
In order to solve the above problems, it has been proposed to modify a phosphor surface by adding a specific organic compound and/or inorganic compound thereto (see JP-B-63-66876, JP-A-63-284290, JP-A-3-273088, JP-B-7-116428, JP-A-1-284583, JP-A-5-28967 and JP-B-44-11769), but it has been found by the present inventors that these proposals do not always provide a phosphor having a satisfactory packing density for forming a phosphor film and preventing a fogging phenomenon.
For example, when a phosphor surface is modified by uniformly treating with an anionic compound, i.e. a negatively chargeable material such as silicon dioxide and acrylic resin as described in JP-B-63-66876 or a nonionic compound such as polystyrene resin as described in JP-B-64-5737, an isoelectric point of zeta potential of the phosphor thus treated can be made about 7 or lower, but a blow-off charge is also lowered, thereby generating a fogging phenomenon.
Further, according to JP-B-63-66876 and JP-A-3-273088, a phosphor surface is modified by treating with a cationic material, i.e. a positively chargeable material such as an alkali metal or alkaline earth metal oxide, a coupling agent having an amino group or the like, and a blow-off electric charge of the phosphor thus treated can be made about 5 .mu.C/g or higher, thereby reducing a fogging phenomenon and improving a color purity and a contrast of the phosphor screen, but an isoelectric point of zeta potential is raised to about 7 or higher, thereby lowering a phosphor packing density for forming the phosphor film and also lowering a luminance.
In the present invention, "blow-off charge" means a blow-off charge generated when contacted with beads of a particle size of from 200 to 800 .mu.m, the surface of which is coated with PVA.